ICRC nurse Hauwa Liman was forced to kneel down with her hands tied, and then shot at close range, local media reports.

A second aid worker has been executed by Boko Haram in Nigeria after a deadline imposed to release imprisoned members of the armed group expired, the government said.

The Nigerian government was "shocked" and "saddened" by the killing of the woman, who was not identified, the information ministry said in a statement.

A clip of the execution of 24-year-old Hauwa Liman was seen on Monday by a correspondent of Nigeria's online news platform, The Cable.

"Liman was forced to kneel down, with her hands tied inside a white hijab which has a crest symbol, and then shot at a close range," The Cable reported.

Liman, a nurse with the International Committee of the Red Cross (ICRC), was kidnapped by the armed group in March along with two other medical workers in Rann, Borno State in the country's northeast.

A midwife with ICRC, Saifura Ahmed, who had been abducted at the same time, was executed by Boko Haram in September.

"We have kept our word exactly as we said, by killing another humanitarian worker, Hauwa Liman, who is working with the International Committee of the Red Cross (ICRC) that were abducted during a raid on a military facility in Rann, Kala Balge in March 2018," the group said in a short statement sent to the Cable.

"Saifura and Hauwa were killed because they are considered as Murtads (apostates) by the group because they were once Muslims that have abandoned their Islam, the moment they chose to work with the Red Cross, and for us, there is no difference between Red Cross and UNICEF," the group added.

"If we see them, we will kill the apostates among them, men or women, and chose to kill or keep the infidels as slaves, men or women."

Plea for mercy

The armed group had earlier released a video message in which it threatened the immediate execution of one of the employees.

The ICRC implored the hostage takers to show "mercy" and release the employees.

The armed group is also holding a 15-year-old schoolgirl, Leah Sharibu, who was abducted in February from her school in the town of Dapchi, Yobe state.

More than 20,000 people have been killed and two million others forced to flee their homes in northeastern Nigeria since Boko Haram launched a campaign in 2009, aimed at forming a breakaway Islamic state.

Over the years, the armed group has kidnapped thousands of adults and children.